Many fire extinguishing systems in use today utilize pressure vessels which are charged with some predetermined weight of a nonsolid fire extinguishing material. When the system outlet is opened, the fire extinguishing material is expelled therefrom under pressure.
Of course, in order to be effective, it is necessary that the system contain, at all times, some predetermined minimum quantity of the fire extinguishing material. In most locations, periodic inspections of the system are made and, at least in the case of portable fire extinguishing systems, the most accurate way of ascertaining whether the system is properly charged is simply to weigh the pressure vessel comprising the extinguisher. In the usual case, the weight of the empty pressure vessel and appurtenances thereto is stamped on the vessel along with the weight of extinguishing material the vessel is to contain. If the weight of the extinguisher does not equal or exceed the desired total, the extinguisher must be recharged in order to meet minimum requirements.
This method of checking is, as mentioned, quite accurate and most likely should be performed periodically regardless of what other measures may be utilized to ascertain whether the charge is at or above minimum requirements. However, it is time-consuming in that it requires transportation of specialized equipment to the extinguisher site to provide for accurate weighing or, in the alternative the movement of the extinguisher to the site of a suitable scale. And, in between periodic inspections there is always the possibility that leakage will occur and/or the extinguisher actually used depeleting its charge, in whole or in part, without being recharged. As a consequence, if a need for the extinguisher arises after such occurrences and before the next inspection, the charge may be insufficient with the result that a fire may not be brought satisfactorily under control.
To alleviate this problem, the prior art has proposed the use of pressure sensing devices in fluid communication with the interior of the pressure vessel for sensing either the pressure of the extinguishing material therein, the pressure of the compressed gas within the vessel utilized to drive the material from the vessel when the vessel is opened, or a combination of both. In some instances where the fire extinguishing material is of the so-called "dry chemical" type, where only a compressed gas pressure is sensed, this worked quite well. However, in other cases, where a nonsolid fire extinguishing material is utilized, and where that material has a vapor pressure that fluctuates widely with temperature, pressure sensing alone is insufficient.
For example, a fire extinguisher normally placed within, say, the engine compartment of a vehicle or a power plant, may reach a temperature of 140.degree. F. or more because of the heat generated within its environment. But this, in turn, will result in a high pressure indication on a pressure gauge which may show to be in a fully charged range on the gauge whereas if the extinguisher were exposed to more typical ambient temperatures, say, 70.degree. F., there would be a clear indication of insufficient pressurization of charge.
To avoid the inaccuracies inherent in pure pressure indications, the prior art has also resorted to the use of pressure gauges which sense the pressure of the interior of the vessel and indicate the same on a scale in both pressure and temperature units. A person inspecting the gauge of such an extinguisher might observe, for example, an indication of 350 psig and a temperature of 70.degree. F. If the observer believes the ambient temperature to be approximately 70.degree. F., he can be assured that the extinguisher is properly charged. However, if at that time, he believes the ambient temperature to be 90.degree. F., because the temperature reading on the pressure gauge is only 70.degree. F., he may deduce that the extinguisher is undercharged.
This system represents an improvement over pure pressure readings, but is also suspect in that it requires a subjective decision on the part of the observer; he must properly estimate the ambient temperature.
Such estimates, in many cases, may be fairly reliable. However, reverting to the example of a fire extinguisher housed in the engine compartment of a vehicle, the observer has no accurate way of estimating the temperature within such a housing and may miss in his estimate by many tens of degrees F. The problem is compounded in that the usual human observer seldom encounters ambient temperatures much in excess of 100.degree. F. and therefore will have very little experience in accurately estimating temperatures that are appreciably higher.